Generally, acoustic waves may be longitudinal waves (propagation in fluids and solids) or transverse waves (propagation in solids and viscous fluids, for example) or combinations of these two types of waves (propagation of surface and guided waves in solids).
At the present time there is a need to increase the operating range of ultrasonic or acoustic transducers and their operating lifetime especially under the physical conditions found in the hot plenum of the main vessel of fast neutron reactors cooled with liquid metal.
This type of transducer may find applications in fast neutron reactors though for this purpose satisfactory operation of the transducers needs to be obtained notably under the following indicative physical conditions:                immersion in liquid metal or alloy (sodium for example);        working temperature under normal conditions: 200° C. (reactor shutdown), 550° C. (reactor operating);        working temperature under incidental conditions: 700° C.;        temperature cycles between 200° C. and 550° C.;        occasional temperature gradient (thermal shock): −20° C./s between 550° C. and 400° C.;        a flux of fast and thermal neutrons and gamma photons;        an operating lifetime of several tens of years (reactor service lifetime: 60 years); and        test or initial conditioning temperatures above the operating temperature (about 600° C. for use at 550° C.).        
These transducers must also be able to operate at room temperature (a few degrees) for laboratory tests.
These transducers must be able to operate as emitters of acoustic or ultrasonic waves, as receivers of acoustic or ultrasonic waves, and as transceivers.
Lastly, these transducers must be able to operate over a wide range of acoustic or ultrasonic frequencies, typically almost continuously up to several megahertz.
Because of their generic features, the improvements to these transducers are also relevant to other fields of applications such as the instrumentation of pressurized-water reactors or indeed even high-temperature instrumentation in non-nuclear industries.
As is known, acoustic waves are generated and/or received using an element made of a piezoelectric or magnetostrictive or electromagnetic acoustic (EMAT) material for converting electrical power into mechanical energy and/or vice versa.
The (piezoelectric, for example) converter element is separated from the propagation medium of the waves by a protective plate (generally made of a metal or a metal alloy) called, in the description, the support plate (sometimes referred to as an interface plate, separating plate, phase plate, coupling plate, front plate, front face, diaphragm, etc.) or more generally the support, which in practice may be integrated into a portion of a housing or a part to be inspected, or of a waveguide.
In order to obtain a satisfactory performance, it is notably recommended to specify:                the choice of the converter material;        the choice of the support material, and more generally of the housing;        the choice and implementation of the type of bond (joint) between the converter material and the support, this bond needing to provide mechanical and acoustic functions, i.e. to be capable of transmitting ultrasound over a wide frequency range (almost continuously up to a few megahertz) and over a wide temperature range (from a few degrees to 550° C., even 700° C.). On its second face, the converter material is coupled to an electrode, this coupling possibly being achieved in the same way as between the converter material and the support, or possibly being achieved differently. Specifically, it may be advantageous for the electrode not to be acoustically coupled to the converter element. The plate may serve as a second electrode. It is necessary for the bond to be compatible with the electrical function of the electrodes (electrode and support, both made of conductive materials), i.e. for it not to introduce between the electrodes and the converter an element the electrical (resistivity) and/or dielectric properties of which may hinder a resistive and/or capacitive, for example, contact electrical coupling between said electrodes and the converter.        
The following assembly “support/first joint/converter material/second joint/electrode”, denoted “assembly” in the rest of the description, must operate (mechanically, electrically and acoustically) durably and have characteristics that are stable under the extreme conditions listed above.
One technique sometimes employed consists in making use of waveguides, one end of which makes contact with the high-temperature medium, the other end, located in a cooler zone that is subjected to a low nuclear flux, bearing a conventional low-temperature transducer. These devices are tricky to implement, notably in the presence of temperature gradients and instabilities.
It will moreover be noted that the so-called “high-temperature” ultrasonic transducers commercially available do not allow the required performance to be achieved with respect to temperature range, frequency range, and operating lifetime. This is because these transducers are notably limited by:                the piezoelectric converter material having an insufficiently high Curie temperature, for example;        the type of joint used between this material and the plate (housing): adhesives, pastes, liquefiable seals, etc. not being able to withstand the temperature experienced or the temperature cycles or gradients, or not being able to withstand the mechanical stresses induced by the temperature or the operation of the transducer, or indeed even causing the transducer to degrade via chemical reaction or attack, etc.; and        the type of joint used between this material and the plate; dry compressive contact (screw, spring) for example is not suitable for transmitting high-frequency ultrasound.        
In addition, the materials used (converter, joint) are often weakened under nuclear radiation conditions.